The thermochromic property of cholesteric liquid crystal compounds has invited considerable effort for their application to thermometer inventions. Many of the liquid crystal thermometers described in the prior art are designed to be used for measuring human body temperature; however, the medical community has demanding needs which have inhibited the commercial success of these thermometers in clinical applications. A medically-acceptable clinical thermometer based on liquid crystal color changes would need to meet the same exacting standards for range (35-41.degree. C.), resolution (0.1.degree. C.), accuracy (0.1.degree. C. in the critical range, 0.2.degree. C. elsewhere), and stability as do mercury-in-glass and electronic clinical thermometers. Furthermore, the thermometers should be designed to (i) measure the temperature at a site which is accepted in the medical community as faithfully representing core body temperature (e.g., rectum, sublingual cavity, or axilla), (ii) be easy to read, clean, and reset between uses, (iii) be safe and comfortable when used at the site, and (iv) retain their accuracy for at least five years during storage and distribution when subjected to temperature extremes of -20 to 60.degree. C. It also is naturally preferred for a thermometer to exhibit competitive advantages such as being less expensive, easier to use, harder to break, more child friendly and require less (preferably no) power than competing products.
Challenges have been encountered in developing thermometers meeting these standards. The liquid crystal forehead thermometer is a good example. Because of its low resolution (.+-.1.degree. C.), it has principally served as a screening device for fever, requiring subsequent confirmation of true body temperature with a mercury-in-glass or electronic clinical thermometer. This is not, however, its only drawback The forehead is an unreliable site for representing core temperature, failing more than 50% of the time to detect fever (false negatives).
Over the years, various compositions and structures have been developed in seeking to improve the performance of liquid crystal thermometers. See, e.g., U.S. Pat. No. 3,974,317, "Thermometric Compositions Including Inert Additives and Products," issued Aug. 10, 1976 to Sharpless (the '317 patent), incorporated herein by reference. The '317 patent describes a liquid crystal system comprising (a) 57.9% cholesteryl oleyl carbonate, 30.7% cholesteryl chloride and 11.4% cholesteryl-n-butoxyphenyl carbonate, and (b) mineral oil. With this composition, temperature differences as small as 0.1.degree. C. can be easily resolved, thus making the composition well-suited for use in clinical thermometers.
This chemistry is itself insufficient, however, for making a clinical thermometer. Constraints are also placed on the heat-sealable substrate and transparent covering film that contain the liquid crystal compositions. See, e.g., U.S. Pat. No. 4,064,872, "Temperature Measuring Device of a Liquid Crystal Laminate," issued Dec. 27, 1977 to Caplan (the '872 patent), incorporated herein by reference. Here it is taught that for the preparation of thermometers useful for medical diagnosis, the separate films comprising the heat-sealable sheet material and the carrier substrate should contain less than 1 mg per square meter of components which will react with the liquid crystals, either during manufacture or storage. The '872 patent teaches use of polyvinyl chloride (PVC) and polyvinylidene chloride (PVDC) coated laminates as the heat-sealing material. This patent emphasizes that the materials selected to enclose the liquid crystal compositions should have a thermal mass that is as low as possible consistent with sufficient durability to allow for repeated use. The '872 patent additionally describes a method for constructing a clinical thermometer from these compositions by arranging them in a dot matrix array.
Thermometers made using the teachings of the '317 and the '872 patents suffer from serious technical shortcomings, however. One such shortcoming relates to readability. The small size of the individual dots (1 mm diameter) and the low contrast between the green liquid state and the gray focal conic state make it difficult to read the thermometer, particularly for those who are unfamiliar with reading the thermometer or who attempt to read it in lighting of low intensity. Because of this deficiency, clinical thermometers made using these teachings have experienced limited commercial success.
Another shortcoming of liquid crystal thermometers relates to the relatively short duration of the signal (15-20 seconds, before reversion begins). When liquid crystal thermometers are removed from one environment in their range of transition to a lower temperature, the signal fades so rapidly that it is difficult to obtain an accurate temperature measurement of the first environment. There are two reasons for this. First, like all thermometers, those made of liquid crystals are of low thermal mass and cool quickly. Second, nearly all liquid crystal compositions respond with time constants of less than one second and thus display exceedingly short memory.
A thermometer structure has been achieved that addresses problems relating to the retention of the signal. See U.S. Pat. No. 5,676,465, "Liquid Crystal Clinical Thermometer," issued Oct. 14, 1997 to Witonsky and Scarantino (the same inventors herein), assigned to the present assignee, which is incorporated herein by reference (hereinafter the '465 patent). The '465 patent discloses a thermometer containing the liquid crystal composition in a pocket or cavity having a domed, inverted structure and an air void comprising about 5 to about 50% of the volume of the cavity. With that structure, a thermometer is provided that retains a signal indefinitely, while at the same time being fully reversible by the application of pressure to the region of the thermometer containing the liquid crystal. The inverted dome structure of the '465 patent also enhances the contrast between the two states, liquid crystal and focal conic, useful for temperature measurement. The inversion of the thermometer pocket may be accomplished by subjecting a thermometer of conventional structure to elevated temperatures.
With the thermometer of the '465 patent, once a reading is taken, the medical practitioner must discard the thermometer, mechanically erase the signal on the thermometer by rubbing his or her finger over the sensor dots, or wait for a period of about one minute. This will induce a change in those clear sensors (black background) with transition temperatures below the temperature of the patient's skin and return them to the colored liquid crystal state. This design has drawbacks in that the nurse or clinical attendant may not continuously monitor temperature during the course of a day. When the thermometer is placed and left on the patient, the temperature information obtained at a later time may not faithfully represent the real-time temperature of the patient, because the thermometer is "peak reading." The temperature information obtained at the time of recording could be several hours old. Consequently, the appropriate intervention action will not be known unless the cleared thermometer is allowed to thermally equilibrate, a process that can require additional minutes of waiting.
As may be appreciated, those involved in the field of thermometry for clinical use continue to seek to develop new thermometer designs that meet the challenging standards posed by the medical community. In particular, it would be advantageous to have a liquid crystal thermometer which is flexible and may be comfortably used on body surfaces that accurately reflect core body temperature. It also would be advantageous to have a liquid crystal thermometer that is capable of continuously providing an accurate measurement of body temperature, without the need for erasing a pre-existing signal. These and other advantages are provided by the instant invention.